diff --git a/src/prime_rl/inference/patches.py b/src/prime_rl/inference/patches.py
index 4f495aa423..3873b79965 100644
--- a/src/prime_rl/inference/patches.py
+++ b/src/prime_rl/inference/patches.py
@@ -18,6 +18,7 @@ def transformers_v5_compat():
     monkey_patch_deep_gemm_ep_scatter()
     monkey_patch_dp_engine_core_pause_resume_deadlock()
     monkey_patch_offloading_connector_cpu_block_count()
+    monkey_patch_routed_experts_capturer_mk_path()
 
 
 @triton.jit
@@ -1086,3 +1087,60 @@ def _patched__post_init__(self: FusedMoEConfig):
         self.is_lora_enabled = False
 
     FusedMoEConfig.__post_init__ = _patched__post_init__
+
+
+def monkey_patch_routed_experts_capturer_mk_path():
+    """Fix RoutedExpertsCapturer.capture() for the modular-kernel (DeepEP) path.
+
+    DefaultMoERunner has two MoE dispatch paths:
+      - naive: dp_size > 1 and not supports_internal_mk -> DP combine concats
+        all ranks' tokens BEFORE select_experts, so topk_ids.shape[0] equals
+        cross-DP total.
+      - modular kernel (MK): supports_internal_mk=True (DeepEP, DEEPGEMM Fp8 MoE,
+        ...) -> DP combine happens INSIDE quant_method.apply, so select_experts
+        sees only this rank's tokens.
+
+    The shipped capture() in vLLM 0.19 hardcodes `assert cumsum[-1] ==
+    topk_ids.shape[0]`, which only holds for the naive path. With DeepEP every
+    DP worker trips the assert during CUDA-graph warmup and engine cores die.
+
+    Mirrors the simpler post-refactor behavior on vLLM main (PR #39917): only
+    slice when topk_ids is the cross-DP concatenation; otherwise copy verbatim.
+    This handles the MK path (local tokens) AND CUDA-graph capture warmup
+    (where dp_metadata still claims max but the captured batch is smaller),
+    both of which would trip the strict either/or check from the earlier
+    upstream fix (PR #37879).
+    """
+    from vllm.forward_context import get_forward_context
+    from vllm.model_executor.layers.fused_moe.routed_experts_capturer import (
+        RoutedExpertsCapturer,
+    )
+
+    def _patched_capture(self, layer_id: int, topk_ids: torch.Tensor) -> None:
+        if self._device_buffer is None:
+            raise RuntimeError("Buffer not initialized. Call init_buffer() first.")
+
+        if layer_id >= self._device_buffer.shape[1]:
+            return
+
+        ctx = get_forward_context()
+        n = topk_ids.shape[0]
+
+        if ctx.dp_metadata is not None:
+            num_tokens_dp = ctx.dp_metadata.num_tokens_across_dp_cpu
+            total = int(num_tokens_dp.sum().item())
+            if total > 0 and n == total:
+                # Naive dispatch: all DP ranks' tokens concatenated upstream.
+                token_num_per_dp = int(num_tokens_dp[self.dp_rank].item())
+                cumsum = torch.cumsum(num_tokens_dp, dim=0)
+                end_loc = int(cumsum[self.dp_rank].item())
+                start_loc = end_loc - token_num_per_dp
+                self._device_buffer[:token_num_per_dp, layer_id, :] = topk_ids[
+                    start_loc:end_loc, :
+                ]
+                return
+
+        # MK path / single-DP / CUDA-graph capture: copy local tensor as-is.
+        self._device_buffer[:n, layer_id, :] = topk_ids
+
+    RoutedExpertsCapturer.capture = _patched_capture